1. Field of the Invention
The present invention generally relates to electrical voltage/current regulators and, more particularly, to a following voltage/current regulator for a motor drive circuit. In a specific application, the motor is used to rotate a scanning mirror in a Forward Looking Infrared (FLIR) system.
2. Background of the Invention
A FLIR scanning system employs an electrical motor to rotate, at a constant speed, a multifaceted mirror to horizontally scan a scene which is focused on one or more infrared (IR) detectors. The typical application for FLIR scanning systems is in aircraft and missiles for military applications. The motors used are generally three-phase, alternating current (AC) motors, and in a specific example, a 1/160 horse power (HP), three phase induction motor is used to drive the horizontal scanning mirror at 60,000 RPM. This motor operates in a vacuum, and the motor current increases as the vacuum level decreases. A one to four minute motor run-up time is required to attain full speed, and during this time there are large inrush currents.
A nominal +28V voltage source conforming to MIL-STD-704C (military standard) is specified for certain applications, including FLIR applications. Wide voltage excursions, a large steady-state voltage range, AC ripple and transient spikes characterize this type of power. Ideally, the scan motor would be driven from a constant +28V source free from all perturbations mentioned above. Additionally, MIL-STD-461A, Notice 4, places very tight limits on conducted emissions in the form of ripple current occuring in the +28V d.c. power line caused by the load, in this case, the motor bridge and the motor.
A power inverter bridge is required to convert the nominal +28V output to the required three phase, AC voltage necessary to drive the motor. Unfortunately, since the MIL-STD-704C voltage source produces fairly large fluxuations in steady state voltages and very large fluxuations in transient voltages, damage to the bridge and the motor can result. For example, this voltage source may have a steady state output which varies in the range of from about 20 to 29 volts, and 87V transients may exist. It is possible to operate the three phase motor, and its three phase inverter bridge, with steady state input voltages from 20 to about 29 volts. For transients over +34 volts, damage to the bridge will result.
Accordingly, it is desired to provide a circuit which will limit the voltage to the motor drive circuit to +29V for steady-state input voltages in excess of this value and to 34 volts for transients Adding to the critical design requirements of the voltage/current regulator is the fact that the FLIR video system is sensitive to electronic switching noise, only a limited volume is available for packaging and a low power dissipation is required.
It is possible to provide an all passive system to supply the necessary voltage to the inverter bridge circuit. However, such a system would require a large L-C filter to meet the ripple requirements, a power Zener diode to protect the motor hybrid for transient voltages over +34V, and a change in specifications to allow 4.6A inrush current. For various reasons, these requirements cannot be met. Alternatively, a switching-type buck or buck-boost regulator with current limiting could be used. However, once again an L-C filter would be required to meet the ripple requirements, the regulator must be synchronized with the horizontal video raster scan of the FLIR system, and shielding would have to be added to prevent video interference.